Aqueous solutions of ammonia are useful as absorbent fluids in a variety of systems, such as absorption refrigeration machines, air conditioning systems, absorption heat pumps, reactifiers, and the like. Many of these systems also include heat exchangers. These systems typically operate under anaerobic conditions at temperatures of up to 450° F. Typically such solutions include ammonia in an amount ranging from about 1 to about 50 weight percent, based on the total weight of the solution.
Aqueous solutions of alkali metal and/or alkaline earth metal hydroxides, such as solutions of calcium-hydroxide, potassium hydroxide, cesium hydroxide and the like and mixtures thereof, are also used in absorbent fluids, for example in absorption cooling machines, chillers, and heat pumps. Typically such solutions include alkali metal and/or alkaline earth metal hydroxide in an amount ranging from about 20 to about 80 weight percent, based on the total weight of the solution.
Although these and other types of absorption solutions can be advantageous, aqueous ammonia, alkali metal hydroxides and/or alkaline-earth metal hydroxides can be corrosive toward the materials used to construct the machines. Such materials can include mild and stainless steel for containment components and copper or copper-nickel alloys for tube bundles (typically for machines in which alkali metal hydroxide and/or alkaline earth metal hydroxide solutions are used), among others.
In addition to the surface damage caused by corrosion, the corrosion reaction evolves hydrogen gas as a byproduct. Incondensibles in the form of atoms or ions can easily enter and diffuse into metals, resulting in the degradation of their mechanical properties under certain conditions.
The severity of corrosion can vary depending upon factors such as temperatures of the system, concentration of ammonia, alkali metal hydroxide and/or alkaline earth metal hydroxide in the absorption solution, metals used in the construction of the unit, the presence of air, and the like. For example, during use, the internal temperature of such machines can be high, typically up to about 450° F. and higher, depending on the type of the absorption cycle, which can increase the corrosive effect of the solution. As noted above, corrosion can generate significant amounts of hydrogen during machine operation, which can adversely impact the performance of the machine.
Various additives, such as lithium chromate, lithium nitrate, and lithium molybdate, have been proposed as corrosion inhibitors in absorption solutions. However, lithium chromate can raise environmental concerns, and its use is being phased out. Further, the level of chromate and its oxidation state must be carefully maintained. Chromate acts as an oxidant. Thus typically large quantities of chromate are added initially and must be replaced periodically to maintain the desired level of corrosion inhibition. If too little chromate is used, then it does not properly passivate the whole metal surface and pitting can result.
Lithium nitrate can potentially evolve ammonia, which can cause stress corrosion cracking of copper based alloys such as heat exchanger tubes. Lithium molybdate exhibits only limited solubility in ammonium, alkali metal hydroxide and/or alkaline earth metal hydroxide solutions. In addition, lithium molybdate is metastable in aqueous ammonium, alkali metal hydroxide, and/or alkaline earth metal hydroxide solutions. Thus it can be difficult to maintain in aqueous ammonia, alkali metal hydroxide, and/or alkaline earth metal hydroxide solutions and to maintain a constant concentration of molybdate ions in solution.
U.S. Pat. No. 5,342,578 reports the use of silicon compounds, in particular sodium silicate, as inhibitors for aqueous ammonium solutions. However, at a certain critical ammonium concentration, the effectiveness of such silicon compounds is diminished. Further, such silicon compounds can have limited solubility in an aqueous medium.
Pending U.S. patent application Ser. No. 08/882,771, filed Jun. 26, 1997, titled “Corrosion Inhibiting Solutions for Refrigeration Systems,” is directed to the use of heteropoly complex anions of transitional elements as corrosion inhibition additives. Refrigeration absorption formulations which included heteropoly complex anions as a component exhibited improved corrosion inhibition properties and low generation of hydrogen gas that is evolved due to the corrosion reaction of alkali halide solutions with the materials of construction of the refrigeration machine. Pending U.S. patent application Ser. No. 08/896,110, filed Jul. 17, 1997, titled “Corrosion Inhibiting Solutions for Refrigeration Systems,” is directed to the use of halide compounds of metallic elements of group Va, and demonstrated that alkali halide formulations containing these elements offer improved corrosion protection and low hydrogen as compared to conventional corrosion inhibitors.